Two-Step Electrochemical Reactions

All the relations reported above are valid for simple two-step electrochemical reactions, when instead of rate constants of the individual steps or of the reaction as a whole, we use the corresponding kinetic parameters ff. We shall assume for the sake 

Electrochemical reactions differ fundamentally from chemical reactions in that the kinetic parameters are not constant (i.e., they are not rate constants ) but depend on the electrode potential. In the typical case this dependence is described by Eq. (6.33). This dependence has an important consequence At given arbitrary values of the concentrations d c, an equilibrium potential Eq exists in the case of electrochemical reactions which is the potential at which substances A and D are in equilibrium with each other. At this point (Eq) the intermediate B is in common equilibrium with substances A and D. For this equilibrium concentration we obtain from Eqs. (13.9) and (13.11), 

At the point of equilibrium, the exchange rate of the reaction as a whole is given by 

With Eq. (13.7), this yields the following relation between the exchange rates  

When the rate of the overall reaction is stated in electrical units [i.e., in terms of the current density (CD) i = nFv], it will be convenient to use the concept of partial current densities of the first and second steps, which are defined as q = l Fv and I2 = IqFvq. In the steady state, v = Vi = y2and i = + I2. With these parameters, Eq. (13.15) becomes 

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Recently anchoring nanosized SnO on anisotropic nanostructures of conducting polymers has attracted much attention, emerging the ID or 2D nanocomposites as high-performed anode materials. Zhang et al. employed a facile two-step electrochemical reaction method including electropolymerization and electrodeposition to fabricate SnO -nanoparticles-decorated PPy nanowires [108]. SEM and TEM reveal that the surface of PPy nanowires was densely coated with SnO nanoparticles (Figure 7.15). 

The zinc/divalent silver oxide cell has a two-step electrochemical reaction ... 

The theory for the reaction of an adsorbed redox couple (2.146) has been exemplified by experiments with methylene blue [92], and azobenzene [79], Both redox couples, methylene blue/leucomethylene, and azobenzene/hydrazobenzene adsorb strongly on the mercury electrode surface. The reduction of methlylene blue involves a very fast two-step redox reaction with a standard rate constants of 3000 s and 6000 s for the first and second step, respectively. Thus, for / < 50 Hz, the kinetic parameter for the first electron transfer is log(m) > 1.8, implying that the reaction appears reversible. Therefore, regardless of the adsorptive accumulation, the net response of methylene blue is a small peak, the peak current of which depends linearly on /J. Increasing the frequency above 50 Hz, the electrochemical... 

When the formal potential of the first step is much more positive than that of the second, AE f < — 200mV (Fig. 7.31c), the intermediate species 02 is stable and two well-separated peaks are obtained, centered on the formal potential of each process and with the features of the voltammograms of one-electron electrochemical reaction. When the A7 ° value increases, the stability of the intermediate decreases and the two peaks are closer, and the transition from two peaks to a single peak is observed (AE —71.2 mV). Eventually, when the formal potential of the second electron transfer is much more positive than that of the first one, AE > 200 mV, the characteristics of the voltammograms are those of an apparently simultaneous two-electron electrochemical reaction (Fig. 7.31a). Note that the... 

In order to analyze the influence of the chemical kinetics on the SWV response of this mechanism when the chemical reaction behaves as irreversible (Keq —> oo), it can be compared with that obtained for a reversible two-electron electrochemical reaction (EE mechanism) at the same values of the difference between the formal potentials of the electrochemical steps, A= E 2 — E (which is always centered atE-mA 1L = (E +E 2)/2). 

Also in the case of Cu.lO", where a [2]catenand-type moiety is present, a two-step protonation reaction occurs, but in this case the luminescence of such moiety is quenched by the Cu-based one [66]. Unfortunately, it is not possible to determine the quenching rate and mechanism for a variety of reasons, including (i) the instability of the protonated forms under laser irradiation in CH2CI2 [56] (ii) the lack of electrochemical potentials of the protonated forms and (iii) the weakness of the MLCT emission band of the Cu-based moiety, which is partially masked by the tail of the emission of the protonated subunit, albeit quenched. However, the energy transfer mechanism is thermodynamically allowed and probably active. 

Such an insertion process occurs via two different steps which can be represented by the following two sequential electrochemical reactions ... 

A thienylplatinum complex with a ferrocenyl group in the ligand 378 shows an absorption band in the visible region. " Reversible two-step electrochemical oxidation reactions of the complex indicate occurrence of electron transfer between Fe and Pt centers of the one-electron oxidized species (Equation (95)). 

The detailed mechanism of battery electrode reactions often involves a series of chemical and electrochemical or charge-transfer steps. Electrode reaction sequences can also include diffusion steps on the electrode surface. Because of the high activation energy required to transfer two electrons at one time, the charge-transfer reactions are beheved to occur by a series of one electron-transfer steps illustrated by the reactions of the 2inc electrode in strongly alkaline medium (41). 

The reaction of 2iac and water is not a simple homogeneous one. Rather it is a heterogeneous electrochemical reaction, involving a mechanism similar to that of a battery. There are two steps to the reaction 2iac dissolves at some locations as shown ia equation 8 while hydrogen gas is generated at other sites. 

The basic electrochemical properties of EMD are summarized schematically in Fig. 2 [1] based on the original work by Kozawa et al. [2-5], The electrolytic Mn02 discharges in two steps in 9 molL"1 KOH. The reaction during each step is shown below. 

In Chapter 6 we considered the basic mles obeyed by simple electrode reactions occurring without the formation of intermediates. However, electrochemical reactions in which two or more electrons are transferred more often than not follow a path involving a number of consecutive, simpler steps producing stable or unstable intermediates (i.e., they are multistep reactions). 

In many electrochemical reactions the individual steps differ in their stoichiometric numbers, in contrast to what was found for reactions of the type of (13.2). A two-step reaction can generally be formulated as... 

In topochemical reactions all steps, including that of nucleation of the new phase, occur exclusively at the interface between two solid phases, one being the reactant and the other the product. As the reaction proceeds, this interface gradually advances in the direction of the reactant. In electrochemical systems, topochemical reactions are possible only when the reactant or product is porous enough to enable access of reacting species from the solution to each reaction site. The number of examples electrochemical reactions known to follow a truly topochemical mechanism is very limited. One of these examples are the reactions occurring at the silver (positive) electrode of silver-zinc storage batteries (with alkaline electrolyte) ... 

With the aid of steady-state polarization and electrochemical impedance measurements in an ammonia containing solution maintained at pH = 9-10.5, Touhami et al. proposed the following mechanism for the H2PO2 reaction utilizing an intermediate adion, Ni,[ds [72] generated in the two step reduction of Ni2+ ... 

The intermediary formation of the Mg-diene complex is confirmed by a two-step reaction method, namely in the first step a solution of 1,3-diene is electrochemically reduced with magnesium electrode in the absence of the ester. After a sufficient amount of electricity is passed, the current is terminated and the ester is added to the solution. The fact that the coupling product is also formed by this two-step method strongly supports the formation of the intermediate Mg-diene complex. 

Metal deposition is an example of a more general class of electrochemical reactions, ion transfer reactions. In these an ion, e.g. a proton or a chloride ion, is transferred from the solution to the electrode surface, where it is subsequently discharged. Many ion-transfer reactions involve two steps. The hydrogen-evolution reaction, for example, sometimes proceeds in the following way ... 

In the past two chapters we have already encountered examples of reactions involving several steps, and introduced the notion of rate-determining step. Here we will elaborate on the subject of complex reactions, introduce another concept the electrochemical reaction order, and consider a few other examples. 

The simplest type of complex electrochemical reactions consists of two steps, at least one of which must be a charge-transfer reaction. We now consider two consecutive electron-transfer reactions of the type ... 

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